1. Field of the Invention
The present invention generally relates to an apparatus and method for starting an electrical generating system. More particularly, the present invention relates to the operation of a switched reluctance generator for generating into a supply system which has no long-term energy storage capabilities.
2. Description of Related Art
The characteristics and operation of switched reluctance systems are well known in the art and are described in, for example, “The characteristics, design and application of switched reluctance motors and drives” by Stephenson and Blake, PCIM'93, Nürnberg, 21-24 Jun. 1993, incorporated herein by reference. FIG. 1(a) shows a typical switched reluctance drive in schematic form, arranged to operate as a motor. The switched reluctance machine 12 is connected to a load 19. The DC power supply 11 can be either a battery or rectified and filtered AC mains or some other form of energy storage. The DC voltage provided by the power supply 11 is switched across the phase windings 16 of the machine 12 by a power converter 13 under the control of the electronic control unit 14. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive, and a rotor position detector 15 is typically employed to supply signals corresponding to the angular position of the rotor. The rotor position detector 15 may take many forms, including that of a software algorithm, and its output may also be used to generate a speed feedback signal. The presence of the position detector and the use of an excitation strategy which is dependent on the instantaneous position of the rotor leads to the generic description of “rotor position switched” for these machines.
Many different power converter topologies are known, several of which are discussed in the Stephenson paper cited above. One of the most common configurations is shown for a single phase of a polyphase system in FIG. 2, in which the phase winding 16 of the machine is connected in series with two switching devices 21 and 22 across the busbars 26 and 27. Busbars 26 and 27 are collectively described as the “DC link” of the converter. Energy recovery diodes 23 and 24 are connected to the winding to allow the winding current to flow back to the DC link when the switches 21 and 22 are opened. A low-value resistor 28 is connected in series with the lower switch to act as a current-sense resistor. A capacitor 25, known as the “DC link capacitor”, is connected across the DC link to source or sink any alternating component of the DC link current (i.e. the so-called “ripple current”) which cannot be drawn from or returned to the supply. In practical terms, the capacitor 25 may comprise several capacitors connected in series and/or parallel and, where parallel connection is used, some of the elements may be distributed throughout the converter.
FIGS. 3(a)-3(c) show typical waveforms for an operating cycle of the circuit shown in FIG. 2 when the machine is in the motoring mode. FIG. 3(a) shows the voltage being applied at the “on angle” θon for the duration of the conduction angle θc when the switches 21 and 22 are closed. FIG. 3(b) shows the current in the phase winding 16 rising to a peak and then falling slightly. At the end of the conduction period, the “off angle” θoff is reached, the switches are opened and the current transfers to the diodes, placing the inverted link voltage across the winding and hence forcing down the flux and the current to zero. At zero current, the diodes cease to conduct and the circuit is inactive until the start of a subsequent conduction period. The current on the DC link reverses when the switches are opened, as shown in FIG. 3(c), and the returned current represents energy being returned to the supply. The shape of the current waveform varies depending on the operating point of the machine and on the switching strategy adopted. As is well-known and described in, for example, the Stephenson paper cited above, low-speed operation generally involves the use of current chopping to contain the peak currents, and switching off the switches non-simultaneously gives an operating mode generally known as “freewheeling”.
As is well known in the art, switched reluctance machines can be operated in a motoring mode, as shown in FIG. 1(a), to drive load 19. In a generating mode, as shown in FIG. 1(b), the load 19 is replaced by a prime mover 19′ to turn the switched reluctance machine and the power supply 11 is replaced with a load 11′ for the generated electricity e.g. a storage battery or a device to be driven.
In the generating mode the phase currents are mirror images (in time) of the motoring currents. Such systems are discussed in, for example, “Generating with the switched reluctance motor”, Radun, Proceedings of the IEEE 9th Applied Power Electronics Conference, Orlando, Fla., 13-17 Feb. 1994, pp 41-47, incorporated herein by reference. FIG. 4(a) illustrates a current waveform when the system is motoring and FIG. 4(b) illustrates the corresponding current waveform for generating. Flux is indicated by the dashed line. It will be seen from FIG. 4(b) that the machine requires a “priming” or magnetizing flux to be established (along with the necessary current to support this flux) before the larger current is returned to the DC link. In other words, some electrical energy is required from the DC link to prime the machine before it is able to convert the larger amount of mechanical energy back to the DC link.
With generating systems which are static (i.e. part of a fixed installation), there is usually a convenient source of energy from which to prime the machine. However some systems are not part of a fixed installation because they are fitted on, for example, marine or automotive equipment, so a special source has to be provided. In systems where the DC link has a relatively low value (e.g. 12V or 48V), it is conventional to incorporate a storage battery 50 in the system, connected across the DC link as shown in FIG. 5(a). This battery is available to provide sufficient energy to prime the phase(s) when the generator is called into action.
In systems where the DC link has a high value (e.g. 300V or more), it is difficult to provide a storage battery at that voltage because of cost and safety implications. Two options have hitherto been available.
Firstly, a low-voltage battery can be used with an up-converter 52, as shown in FIG. 5(b). This overcomes many of the safety problems associated with isolation of a high-voltage source, but is costly. In addition, unless the up-converter is bi-directional, some other form of battery charging must be provided to re-charge the battery, entailing further cost.
Secondly, the system can rely on the short-term energy storage provided by the DC link capacitors in the power converter. While this will be successful if the amount of charge left in the capacitors at the time of starting is sufficient to energize the machine adequately, there is no guarantee that the capacitors will hold their charge during a prolonged shutdown. Further, it is often a requirement that capacitors are discharged before any maintenance work is done on the converter, and this would preclude subsequently starting the system by this method.
There is therefore a need for economically starting a generating system on a bus which has no long-term storage.